A lot of new cars have fancy cameras on the rear bumper to help you parallel park. If your car doesn't have that feature, DIY blog Mad Science has put together a tutorial to roll your own sensor system with an Arduino.

The build is incredibly simple and you only need a ping sonic range finder, Arduino, a buzzer, and just a few wires. The sonic range finder emits a sound wave that tells you when you're close to obstacles behind your car. When you're too close to an object the buzzer sounds. If you're not so good at parallel parking or you just want a warning when someone's tailgating you this should help. Find the full guide to making it for yourself over on Mad Science.